1. Field of the Invention
This invention regards a method of pressure controlled sequential operation. More particularly, it concerns a method of controlling a sequence of operations in a downhole tool. The invention also comprises a device for implementing the method.
2. Description of the Related Art
When working under ground, e.g. in a borehole, it is vital that the order of a sequence of individual operations may be controlled in a reliable manner.
It is known to use telemetry and rotational frequencies to communicate from the surface and down to the downhole tool in, for example, a pipe string. The use of electrical signals for such communication is also known.
These forms of communication have shortcomings that to a considerable extent reduce their applicability, as telemetry requires the use of relatively sensitive instrumentation, and the use of rotational frequencies is dependent on the downhole tool being rotatable. Electrical conductors are often exposed to damage.
Thus it has become more common to control tools through variation of the working fluid pressure, for example during coiled tubing operations, where a relative increase in the pressure of the working fluid may be used to initiate an additional operation.
For complex operations that require the use of a large number of relatively sensitive valves, and where the pressure interval between the opening of one valve in the sequence and the opening of the next is small, it has been found that, due to a reduced operational safety in the valve system, working fluid is unsuited for use in valves of this type.
A further adverse effect of pressure controlled sequential operation is that the remaining tool functions, where use is made of e.g. hydraulic cylinders, are often influenced by pressure variations in the working fluid. Moreover, it has been found that the maximum pressure of the working fluid is often too low to allow the execution of certain operations connected with a downhole tool.
The object of the invention is to remedy these disadvantages.